The present invention relates to an overload protection arrangement for an electric motor in general, and more particularly for use in hand-held power tools.
There are already known various overload protection arrangements for electric motors, in which a temperature sensor is provided which supervises or detects the temperature of the winding of the electric motor which is in danger of being overloaded under certain operating conditions of the electric motor. When this sensor detects the rise of the temperature of the winding above an acceptable level, it issues a signal which is used for reducing the supply of power to the electric motor.
So, for instance, it is known to use a positive temperature coefficient resistor as the temperature sensor, this so-called PTC-resistor being arranged in the affected winding of the electric motor, and to use the signal generated thereby when the acceptable temperature level has been exceeded for shutting off the tool driven by the overloaded electric motor by means of suitable devices, such as a relay or an electronic control device.
There are further known excessive current protection switches by means of which the voltage supplied to the electric motor of the power tool is reduced to a constant value when the danger of overheating of the winding of the electric motor exists, in order to render it possible for the electric motor to still run so as to assure the cooling of the power tool during the continuing idle running of the electric motor. However, if the electric motor or the tool is still being loaded after the voltage has been thus reduced, there exists the danger of blocking of the rotor of the electric motor, as a result of which the opposing voltage is reduced and the electric current increases again, so that the winding in question can still burn out. An arrangement of this type is disclosed, for instance, in the European Patent Application No. 7617.
It is further known to shut off the electrically powered tool by means of an overload or excessive current protection switch in the event of danger of winding overheating. However, as a result of the complete shut-off of the power tool, the stream of cooling air is interrupted, so that the cooling continues only at a very slow pace, so that the time span during which the power tool is idle prior to the moment when the operation thereof can be safely recommenced is relatively long. This, of course, is also very disadvantageous, even though not as much as the possibility of burning out of the affected winding which exists in the arrangement discussed above.